Method of manufacturing coiled heating element

Abstract

A coiled heating element is manufactured by providing a coiled element of bare resistance wire in which the coil pitch is not greater than twice the cross-sectional dimension of the wire. The element is stretched by an amount permitting recovery to its original configuration and an electrically insulating particulate material is applied to the stretched element. The stretched element is then allowed to return towards its original configuration thus trapping particulate material between adjacent coils of the element and the element is heated so as to oxidize the surface of the wire.

We claim: 
     
       1. A method of manufacturing a coiled heating element, which method comprises the steps of: providing an unsheathed coiled element of bare resistance wire, the wire having a cross-sectional dimension and a surface, and the coiled element having a coil pitch not greater than twice the cross-sectional dimension of the wire;   stretching the element by an amount permitting recovery to its original configuration;   applying to the element an electrically insulating particulate material;   allowing the stretched element to return towards its original configuration thereby trapping particulate material between adjacent coils of the element; and   heating the element so as to oxidize the surface of the wire.   
     
     
       2. A method according to claim 1, wherein the particulate material is an oxide. 
     
     
       3. A method according to claim 2, wherein the particulate material is selected from the group consisting of aluminium oxide and magnesium oxide. 
     
     
       4. A method according to claim 1, wherein the particulate material is a refractory clay. 
     
     
       5. A method according to claim 4, wherein the particulate material comprises china clay. 
     
     
       6. A method according to claim 1, wherein the particle size is less than the cross-sectional dimension of the wire. 
     
     
       7. A method according to claim 6, wherein the particle size is less than one tenth of the cross-sectional dimension of the wire. 
     
     
       8. A method according to claim 1, wherein the particle size of the particulate material is from 30 to 50 microns. 
     
     
       9. A method according to claim 1, wherein the particulate material is applied to the element by dipping. 
     
     
       10. A method according to claim 1, wherein the particulate material is applied to the element by spraying. 
     
     
       11. A method according to claim 1, wherein the particulate material is in the form of a dry powder. 
     
     
       12. A method according to claim 1, wherein the particulate material is in the form of a slurry. 
     
     
       13. A method according to claim 1, wherein the element is heated to a temperature of 1100° to 1200° C. 
     
     
       14. A method according to claim 1, wherein the heating is effected by placing the element in a furnace. 
     
     
       15. A method according to claim 1, wherein the heating is effected by passing an electric current through the element.